An epitaxial wafer has been widely used from long ago as a wafer for fabricating a discrete semiconductor, a bipolar IC and the like, because of its excellent characteristics. Especially, a low-resistivity wafer for epitaxial growth, whose resistivity is lowered, which is applied to a substrate of an epitaxial wafer, has grown in importance, because of excellent latchup characteristic and gettering ability. For these reasons, a tendency for lowering resistivity of an Epi-Sub (Epitaxial-Substrate) single crystal as the wafer for epitaxial growth has been progressed in recent years.
A silicon wafer used for a semiconductor device is generally produced from a silicon single crystal grown by mainly the Czochralski method (hereafter, referred to as “CZ method”). CZ method is a method by which a seed crystal is contacted with a silicon melt contained in a quartz crucible, the seed crystal is pulled up, and thereby the silicon single crystal is grown under the seed crystal. In the growth of the silicon single crystal by CZ method as described above, dislocations are generated in the seed crystal in high density as a result of thermal shock at the time that the seed crystal contacts with the silicon melt. For this reason, in order to eliminate generated dislocations, the steps of: a neck (a necking portion) is formed; the diameter of the crystal is increased to a desired diameter; and thereby the silicon single crystal is formed, are necessary when the seed crystal is pulled up. The above-mentioned method of forming the necking portion by decreasing a diameter of the seed crystal is widely used as the Dash Necking method and is sometimes called as a necking step.
In the step of decreasing the diameter of the seed crystal by the Dash Necking method described above, the diameter of the necking portion of the seed crystal is conventionally 4 mm or less, and a growth rate is controlled so that the diameter may become such a predetermined value and conventionally varied from 1 to 10 mm/min.
However, when the above-mentioned Epi-Sub single crystal having low resistivity (e.g., 0.1 Ω·cm or less) is grown, since a concentration of dopant impurities such as metal boron for doping is high, a frequency of generating dislocations during growth is higher than that of a silicon single crystal having ordinary-resistivity, therefore there has been a problem of difficulty in single-crystallization (e.g., Japanese Patent Application Laid-open (kokai) No. 2004-315258).
Further, in the case of an Epi-Sub single crystal having an extremely low resistivity of 1.5/1000 Ω·cm, since dopant impurities with high concentration in the crystal inhibit migration of dislocations, it is difficult to eliminate dislocations completely even if the above-mentioned necking step in the process of growth of the silicon single crystal by CZ method is performed. Moreover, as for a crystal which is judged that the single crystal can be formed by CZ method by observation of the crystal surface, there is a problem that defects such as scratches are generated at a wafer surface (FIG. 1) when such a low-resistivity single crystal ingot is processed into a wafer.
As for the above-mentioned low-resistivity single crystal ingot from which the wafer is produced, although defects are not observed at the surface, defects such as scratches are detected in some cases at an end face of the ingot by observation using X-ray topograph after the ingot is cut and the end face thereof is etched (FIG. 2). Moreover, even if an ingot in which scratches are not observed by X-ray topograph is processed into a wafer, defects as shown in FIG. 1 are detected in some cases at the wafer. That is, as for the above-mentioned Epi-Sub single crystal having low resistivity, it has been very difficult to detect defects such as scratches at the state of ingot, and the judge method thereof has not been established yet. Therefore, since it is impossible to confirm whether defects such as scratches present or not until processing into the wafer, there have been problems that a cost for wafer processing is high and the productivity is decreased.